Chemical bath deposition (CBD) method has bend developed as a well-known thin film technique, which was first announced by the Boeing Company in 1982. Advantages of the chemical bath deposition technique include easy implementation, low equipment cost, and high quality coating, etc. Conventionally, when implementing the chemical bath deposition process to prepare a thin film, the working piece will be vertically placed in the plating container where the chemical solution is heated. It should be understood, however, that thermal field and flow field deposition in the chemical bath container may directly affect uniformity of the coating layers. Therefore, the thermal field of the chemical bath container and the flow field of the reaction solution must be precisely controlled.
Generally, there are two main nucleation mechanisms during the chemical bath deposition process: homogeneous nucleation and heterogeneous nucleation. Heterogeneous nucleation occurs when anions and cations in the solution react to formations of nuclei in a heterogeneous interface. After proceeding subsequent chemical ion reactions, the nuclei continue to stack and grow and transforms into a thin film on the heterogeneous interface, wherein the heterogeneous interface can be a solid-liquid interface or a gas-liquid interface. On the other hand, homogeneous nucleation occurs when anions and cations directly react into nuclei in solution. After proceeding subsequent chemical ion reactions, the nuclei continue to stack and grow and transforms into suspended particles in the solution.
For conventional chemical bath deposition process, the presence of suspended particles is an important problem to overcome. The main problem is that the suspended particles may attach to the thin film during plating process, undermining the uniformity of film thickness. For example, when a large area thin film is prepared using vertical chemical bath deposition, a higher amount of suspended particles may occur at the bottom of the chemical bath container, affecting uniformity and surface smoothness of the coating layer in the bottom of the chemical bath container.
According to the abovementioned discuss, the conventional chemical bath deposition process has two major problems which must be overcome, namely; uniformity of thermal field distribution and deposition of suspended particles. Particularly, during deposition of large area thin films, these two effects may become more apparent.
In order to address the issue of thermal field distribution, U.S. Pat. No. 7,541,067 and U.S. Pat. Publication Nos.: 2009/0246908, 2009/0255461, 2009/0223444, the entirety of which are incorporated herein by reference, disclose a chemical bath deposition (CBD) method with the face-up placement of the substrate which directly heats the substrate instead of the solution to address the issue of the thermal field distribution. FIG. 1 is a schematic view illustrating a conventional chemical bath deposition apparatus. In FIG. 1, a chemical bath deposition apparatus 10 includes a chemical bath deposition section 11 where the reaction solution flows from an inlet port 13 to an outlet port 14. A substrate 18 is disposed at the bottom of the chemical bath deposition section 11. The surface 18A of the substrate 18 is arranged faced upward. The reaction solution in the chemical bath deposition section 11 was maintained at temperature within a range between 55-80° C. The substrate 18 is heated up by a heater 19. A cooling device 29A is disposed on the top of the chemical bath deposition section 11 constructed as a cold-wall reactor. Chemical compound thin film deposited on the top surface 11A of the chemical bath deposition section 11 can thus be prevented.
The deposition method implemented by the chemical bath deposition apparatus 10 is advantageous in that less reactive solution can be consumed to achieve the same thin film quality. Although the abovementioned method can successfully address the thermal field distribution and flow field distribution issues, deposition of the suspended particles in the solution cannot be effectively avoided. Large quantities of suspended particles may be deposited on the substrate surface, affecting the coated thin film quality.
There are some prior techniques which reduce the solution temperature to avoid occurrence of the suspended particles and polishing and flattening the surface of the coating layer after completion of thin film deposition. Reducing the solution temperature, however, would result in increased deposition time. Accordingly, a chemical bath deposition apparatus is eagerly needed to effectively address the issue of thermal field distribution and to effectively reduce and prevent the suspended particles deposited on the substrate surface.